The invention relates to digitizing systems and techniques including pointing devices such as cursors and pens that transmit carrier signals which are received by a grid of X and Y conductors in a platen surface to produce digital data representative of cursor or pen location, and more particularly to improved techniques for reducing or eliminating effects of high levels of ambient noise.
The prior art of digitizing tablets includes a wide variety of system approaches which typically include transmission of a carrier signal from a pointing device. The pointing device can be cordless or connected by a cord to a suitable carrier signal generator. The transmitted carrier signal induces carrier signals in X and Y grid conductors. Position information of the pointing device is determined by scanning the X,Y grid conductors, and sensing, amplifying, and processing the amplitudes of the scanned X and Y grid conductors to derive data corresponding to the present position of the pointing device. The prior art also includes systems that transmit from the grid in a time scanned fashion, rather than transmitting from the pointing device. Such systems receive the signal in a single antenna located in the pointing device.
Both types of prior digitizing systems are susceptible to errors caused by electronic interference generated nearby and radiated into the antenna system of the digitizing tablet. This is particularly true if the digitizing system is implemented using electrostatic means of signal transmission, since the antenna impedances are higher in such systems and are more subject to receiving larger noise signals in the presence of radiated noise. Although electrostatic digitizing systems generally utilize much smaller signals and less power than magnetic digitizing systems, particularly for cordless pointing devices, magnetic digitizing systems nevertheless do "pick up" ambient electrical noise, and digitizing performance can be degraded. Any noise that is superimposed upon the pointing device signals or the resulting X and Y grid conductor carrier signals is likely to produce digitizing errors or command errors. One common source of high noise interference is computer monitors. Recently available monitors generate high levels of noise that is in the spectrum of operation of digitizing systems. Modern high resolution computer monitors generate noise in the spectrum of about 15 kilohertz to 150 kilohertz. (This frequency band is not protected by RFI emission regulation requirements of the Federal Communications Commission.) Since there are no such RFI emission limits, it is becoming increasingly difficult for digitizing systems to find an interference-free spectrum in which to operate a digitizing tablet. The monitor noise causes amplitude modulation of the signals received from the pointing device. Since amplitude information of the scanned X,Y conductors is used to derive X and Y coordinates of the pointing device, the amplitude modulation can introduce errors into the pointing device position data. Consequently, the noise produced by the monitor 5 usually manifests itself as "jitter" in the pointing device position data.
Prior methods for reducing the effects of such high noise interference include increasing the transmission power of the digitizing system or finding an interference-free frequency band for operating the digitizing system. However, increasing power of transmission of the digitizing system generally is undesirable because of the accompanying increased circuit complexity and increased power consumption of the system, and interference-free frequency bands are becoming less and less available.
FIG. 1 illustrates the above problem, wherein a typical prior digitizing tablet 1 operates in conjunction with a pointing device 2 which transmits electrostatic signals that are detected by X and Y grid conductors such as 3. A high resolution monitor 5 generates electrostatic and electromagnetic noise 6 which induces noise signals in the various X and Y grid conductors 3. Grid conductor scanning circuitry (not shown) multiplexes a selected grid conductor to a signal conductor 4, which is connected to an input of a preamplifier 8A which is referenced to a ground conductor 9. Reference numeral 7 illustrates a typical waveform appearing on conductor 4, including the detected high frequency carrier signal and with a low frequency modulating envelope 7A produced by the noise 6. Both the detected grid conductor carrier signal and the noise-produced modulation appear on the output conductor 10, which is input to suitable analog-to-digital conversion circuitry and amplitude processing circuitry to generate X,Y coordinate data for pointing device 2 and/or frequency discriminated command signals produced in response to switches on pointing device 2. Numeral 11 shows the signal on amplifier output conductor 10, containing both the digitizer carrier signal and the noise-induced modulation 7A.
It is believed there would be a very large market for a low cost graphics input system that could operate in a high-noise environment, especially such a system that would be suitable for use in tablets or digitizing systems utilizing increasing popular cordless pointing devices.